Using a repeater or relay (hereafter “repeater” for convenience) is a cost-effective way to increase coverage in a mobile communications system. A repeater extends the coverage of a base station, that is, the size of a cell, by retransmitting the signal received from the base station. However, present technology repeaters do not fully deliver, to cell edges, on the promise of increased data rates and capacities available in systems such as Long Term Evolution (LTE) Advanced because of issues such as self-interference and inefficient bandwidth utilization.
Layer 1 repeaters do not decode signals but perform an amplify-and-forward operation. Some layer 1 repeaters may perform other simple low layer functions such as filtering and beamforming. These repeaters only have layer 1 (i.e., physical layer) functionality.
One drawback of prior art repeaters is self-interference. Self-interference can occur with On-Frequency Repeaters (OFR) and with other repeater nodes. Self-interference occurs because the transmit and receive antennas are insufficiently isolated. Thus, some part of the output signal is received at the input. To avoid self-interference, self-interference cancellation techniques can be used. However, in high gain repeaters, the achievable antenna isolation may nevertheless not be sufficient. Multiple repeaters may also interfere with each other. That is, the signal forwarded by one repeater node may be received and amplified by another repeater node where it is seen as interference.
Some of these drawbacks may be mitigated by using static Frequency Translating Repeaters (FTR). An FTR may receive a WCDMA input signal on one radio channel, for example, shift the signal in the frequency domain by a fixed amount to another radio channel, and transmit the shifted WCDMA signal. Such prior art repeaters do not distinguish between different users in a multi-user signal (in the downlink, a multi-user signal is transmitted from a single source and represents different users' data, whereas in the uplink, a multi-user signal is the superposition of multiple users' signals received from multiple sources). The amount of frequency shift or offset is configurable, but static. That is, the frequency shift amount is not changed while the FTR is repeating.
The LTE specification and its successors such as LTE-Advanced use Frequency Division Multiplexing (FDM) and Frequency Division Multiple Access (FDMA) to package multiple users' data in the downlink and uplink, respectively. This is in contrast to, for example, WCDMA, where code division is used to handle multiple users. More specifically, in the downlink of LTE and LTE-Advanced, Orthogonal Frequency Division Multiplexing (OFDM) is deployed whereas, in the uplink, Discrete Fourier Transform Spread-OFDM (DFTS-OFDM, a precoded version of OFDM) is applied. In OFDM, spectrum is partitioned into many narrowband subcarriers and 12 consecutive subcarriers are grouped into a basic scheduling unit called a Resource Block (RB). One user can be scheduled on one or multiple RBs, which may be consecutive or not.
Prior art FTRs treat an incoming multi-user signal as a single signal entity and do not differentiate between different users. This means that signals belonging to different users are shifted equally in frequency, which leads to sub-optimal resource usage since different users experience different interference and would benefit from being served at different frequencies. Furthermore, the frequency shift amount in prior art repeaters is a static parameter which is not changed during operation. Since interference as well as scheduling and user allocation are dynamic processes that vary, for example, with traffic demand, mobility, and channel changes, static FTRs perform only sub-optimally.
The problem with different time and spatially varying interference situations for users exploiting a repeater with a static frequency translation is illustrated in FIG. 1. In FIG. 1, a base station 100 transmits a multi-user signal 110 which packages signals for users MS1 and MS2 at adjacent frequencies f1 and f2, respectively, as shown in base station resource allocation 120. Static Frequency Translating Repeater 150 receives multi-user signal 110, does not unpackage any user's signals, and simply shifts packaged multi-user signal 110 in frequency to multi-user signal 160—which comprises the packaged signals for users MS1 and MS2 at adjacent frequencies, albeit at frequencies f3 and f4, respectively, as shown in FTR resource allocation 170. At timed, MS1's receiver experiences interference (“H”) on frequencies f2 and f4 and no interference (“L”) on frequencies f1 and f3 as illustrated in reference 181-0. Reference 182-0 indicates that MS2's receiver experiences interference only at frequency f4 at timed. However, because MS2 is allocated frequency f4, MS2 experiences signal loss at least at time0. At time1, the interference environments change for MS1 and MS2 as shown in references 181-1 and 182-1 and neither user experiences signal loss at their allocated frequencies. Because prior art FTR 150 is unable to separate the users' signals and dynamically allocate frequencies on a user-by-user basis and unable to dynamically select a frequency shift amount, FTR 150 cannot redirect MS2's signal to a low-interference frequency to avoid the high-interference at frequency f4.
Static FTRs also lead to inefficient bandwidth utilization in mobile communications systems employing frequency hopping. Typically, in a frequency hopping system, it is desirable that the input and output spectra of the FTR do not overlap. Thus, with prior art repeaters, the hopping pattern can at the most extend over half the available system bandwidth since the other half is needed for the translated signal. This limits both resource usage as well as frequency hopping gain since the maximum hop distance is limited.
Thus, it is desirable to have a frequency translating repeater which overcomes at least some of the aforementioned disadvantages.